Due to its good thermal conductivity, hexagonal boron nitride powder may be used as a filler for polymers in applications which simultaneously require good electrical insulation capability of the filler that is used. Furthermore, boron nitride powder is also used as a sintering powder for hot pressing in applications in metallurgy. Hexagonal boron nitride powder is moreover used in cosmetic preparations, as a lubricant, as a release agent in metallurgy and as a raw material for producing cubic boron nitride.
Hexagonal boron nitride powder is technically synthesized by nitridation of boric acid in the presence of a nitrogen source. Ammonia may be used as the nitrogen source, in which case calcium phosphate is typically used as a carrier material for the boric acid. An organic nitrogen source, such as melamine or urea, may also be reacted under nitrogen with boric acid or borates. Nitridation is typically carried out at temperatures of 800 to 1200° C. The boron nitride thus obtained is largely amorphous and is also referred to as turbostratic boron nitride. From the amorphous boron nitride, hexagonal, crystalline boron nitride is produced at higher temperatures of up to about 2100° C., preferably in a nitrogen atmosphere. For this high-temperature treatment, crystallization additives are also frequently added to the amorphous boron nitride.
Hexagonal boron nitride (hBN) in the form of primary particles having a platelet morphology develops during the high-temperature treatment. Typical platelet sizes range from approximately 1 to 20 μm; but platelet sizes of up to 50 μm and more are possible. After production, the annealed mass is usually milled or de-agglomerated in order to obtain powders capable of being processed.
The thermal conductivity of hexagonal boron nitride is greater in the plane of the platelet (a-axis) than perpendicular thereto (c-axis). In the direction of the c-axis, the thermal conductivity is 2.0 W/mK; by contrast, in the direction of the a-axis, it is 400 W/mK (see R. F. Hill, SMTA National Symposium “Emerging Packaging Technologies,” Research Triangle Park, N.C., Nov. 18-21, 1996).
In addition to platelet-shaped primary boron nitride particles or agglomerates of such primary particles which accumulate during the synthesis of hexagonal boron nitride, hexagonal boron nitride powder is also frequently used for filler purposes in the form of specifically produced granules, i.e. in the form of secondary particles built up from the primary particles. Granulation improves the processing properties, such as flowability and dosing of the boron nitride powder, and it is possible to achieve higher filler loadings and higher thermal conductivities, for example in polymer-boron nitride composite materials. In doing so, there are different methods for producing such secondary particles that lead to granules having different morphologies and different properties.
For the specifically produced granules, the term “agglomerates” is often used; the term is likewise used for the agglomerates or aggregates accumulating due to manufacturing conditions during the synthesis of hexagonal boron nitride.